The present invention relates to a power conversion system for converting d.c. power into three-phase a.c. power, and more particularly to a power conversion system which needs isolation between the input and the output, a method for controlling the same, and an uninterruptible power supply using the same.
Conventionally, an uninterruptible power supply has been known as a power conversion system which needs isolation between the input and the output. For example, a power conversion system disclosed in JP-A-61-236371 has been arranged to temporarily convert direct current into a higher frequency than that required by a load by using an inverter, isolating the converted current in a transformer, and then converting it into a desired frequency, for the purpose of reducing the size and weight of the transformer.
The aforementioned power conversion system, however, provides only single-phase a.c. power. To obtain three-phase a.c. power, therefore, it is necessary to provide three such systems. That is to say, the power conversion system has a shortcoming that it becomes larger and heavier if three-phase a.c. power is required.